Major Logging Overhaul

in this commit:

- centralize logging output handler.
- set global Info/Debug/Error log levels based on config file or flags.
- remove per-plugin debug arg handling.
- add a I!, D!, or E! to every log message.
- add configuration option to specify where to send logs.

closes #1786

plugins/inputs/system/processes.go

@@ -118,7 +118,7 @@ func (p *Processes) gatherFromPS(fields map[string]interface{}) error {
 		case '?':
 			fields["unknown"] = fields["unknown"].(int64) + int64(1)
 		default:
-			log.Printf("processes: Unknown state [ %s ] from ps",
+			log.Printf("I! processes: Unknown state [ %s ] from ps",
 				string(status[0]))
 		}
 		fields["total"] = fields["total"].(int64) + int64(1)
@@ -169,14 +169,14 @@ func (p *Processes) gatherFromProc(fields map[string]interface{}) error {
 		case 'W':
 			fields["paging"] = fields["paging"].(int64) + int64(1)
 		default:
-			log.Printf("processes: Unknown state [ %s ] in file %s",
+			log.Printf("I! processes: Unknown state [ %s ] in file %s",
 				string(stats[0][0]), filename)
 		}
 		fields["total"] = fields["total"].(int64) + int64(1)
 
 		threads, err := strconv.Atoi(string(stats[17]))
 		if err != nil {
-			log.Printf("processes: Error parsing thread count: %s", err)
+			log.Printf("I! processes: Error parsing thread count: %s", err)
 			continue
 		}
 		fields["total_threads"] = fields["total_threads"].(int64) + int64(threads)